The first defense witness called in Nicole Marie Beecroft’s murder trial testified Friday that how Beecroft’s baby died during an unattended birth in 2007 couldn’t be determined.

Dr. Mary Ann Sens said there was no definitive proof of live birth before Beecroft stabbed the newborn girl 135 times. Her testimony countered that of prosecution witnesses, including the medical examiner who performed an autopsy, who said the baby was born alive and died from stab wounds.

“Frankly, people are too cavalier to call one thing or another in my opinion,” Sens said of the difficulty at concluding a cause of death. She is a pathology professor at the University of North Dakota Medical School and a practicing medical examiner in Grand Forks, N.D., and six northwestern Minnesota counties.

Beecroft, now 24, has spent seven years in jail and prison on a life conviction without parole for killing her baby. Her new trial was ordered by the Minnesota Supreme Court last summer on grounds that two people not associated with the first trial had influenced testimony.

Beecroft was living in Oakdale with her mother when she gave birth to the baby and disposed of the girl’s body in a trash can outside her house. The 17-year-old Tartan High School senior had disguised her pregnancy and was alone during the birth.

Sens, under questioning from defense attorney Christine Funk, countered many of the arguments made a day earlier by Dr. Kelly Mills, the Ramsey County medical examiner who performed the autopsy on the baby’s body.

Mills said in testimony Thursday that she found air in the baby’s lungs, evidence of a live birth.

But Sens said that she has found aerated lungs in stillborn babies. She also said that what appears as air might be bacterial gas, water and amniotic fluid.

“I would say there’s no definitive evidence of air in the lungs,” she said after reviewing microscopic slides provided from the autopsy.

Sens also refuted that bruising on the girl’s body suggested a live birth, as Mills had testified.

“One can’t say. In and of itself we can’t say this is a vital reaction,” Sens said.

Prosecutor Siv Yurichuk, in cross-examination, pressed Sens on which doctor would be the greater authority in interpreting details of a death.

Sens responded that the pathologist who performed the autopsy “generally has the best firsthand knowledge.” In response to a question from Yurichuk about the baby’s finger moving after birth, Sens replied: “If a body part moved that’s one of the signs of life.”

Testimony continues Monday in Washington County District Court before Chief Judge John Hoffman.